Field of the Invention
The present invention relates to moisture-curable single-component (1K) topcoat coatings.
Description of the Prior Art
The U.S. Navy's predominant topside coatings are haze gray semi-gloss silicone alkyds. These coatings have been used on the topsides (freeboard and superstructure) of surface ships by the Navy since the early 1960s. Silicone alkyd coatings are considered “user friendly” in that they are single-component (all-in-one can) paints that have an indefinite pot-life in a closed can, have been reformulated to maintain compliance with volatile organic compound (VOC) limits, and will cure even under the most adverse conditions. Unfortunately, these user-friendly paints have several inherent limitations, which include color fading, chalking, loss of gloss, limited resistance to shipboard hydrocarbons, and limited surface hardness that makes running rust and soot staining extremely difficult to remove. In addition, peeling, cracking and delamination of cured silicone alkyds can often result due to application over inadequately prepared surfaces.
Silicone alkyd coatings can be formulated as single-component (1K) systems because they contain unsaturated fatty acid groups that crosslink in the presence of atmospheric oxygen. The coatings do not begin to cure until they are applied to a surface and the solvent evaporates, thereby possessing essentially a limitless pot-life in a closed can. For Navy ships, silicone alkyd topside coatings are specified as a Haze Gray color with a semi-gloss finish, are available in a variety of volatile organic compound (VOC) levels (e.g., 340 g/L, 250 g/L), and have a service-life of approximately 6-12 months. Frequently, silicone alkyd coatings need to be touched-up or repaired (e.g., via roller or brush), yet this mundane task would not be required if silicone alkyd coatings did not easily fade, discolor, peel/delaminate or stain within a few months after application. A single application of silicone alkyd is specified at 2-5 mils dry film thickness (DFT); however, due to the constant over-coating for maintenance, it is not uncommon for surfaces to possess greater than 50 mils of topside coating.
Although Navy surface ships utilize silicone alkyd topcoats, the majority of topcoats used by the Navy are polyurethanes. Polyurethanes are formed by reaction of an isocyanate-functional material with a hydroxyl-functional material (e.g., polyester polyol or water), and are used to provide protective camouflage, exterior color stability, flexibility, chemical warfare agent resistance, hydrocarbon resistance and chemical resistance. Polyurethane topcoats can be two-component systems or single-component systems. Polyurethane topcoats contain toxic isocyanates that can cause serious health issues for both coating applicators and the environment, and non-isocyanate alternatives that offer equal or greater performance are of high interest. Furthermore, two-component coatings require the mixing of components before application, which can result in insufficient cure times, reduced hardness, poor adhesion, and poor appearance if applicators do not mix the materials correctly. Two-component coatings also have a limited pot-life, which is an issue for individuals performing touch-up and repair applications. For these reasons, single-component coatings are favored over two-component systems.
Polysiloxane-based coatings have an inherent durability advantage over traditional organic-based materials due to the presence of silicon-oxygen bonds. The Si—O bond, which has a bond enthalpy of 110 kcal/mol, is stronger than the carbon-hydrogen (99 kcal/mol) and carbon-carbon (83 kcal; mol) bonds found in organic coatings, thereby leading to an increase in thermal stability and resistance to oxidative degradation by sunlight. Polysiloxanes, like many silicon-based materials, are relatively non-toxic to humans, especially when compared to the health issues associated with isocyanate-containing materials.
Two-component (2K) polysiloxane coatings are based on materials that contain both reactive organic groups and moisture-curable alkoxysilane groups. These coatings are often referred to as “hybrid cure coatings,” where one portion of the coating is crosslinked by the ambient reaction between organic groups, such as amines and epoxies, while the other portion forms a siloxane network via moisture hydrolysis of the alkoxysilane groups and condensation of the resulting silanols. These coatings offer good exterior durability, hardness, chemical resistance, and direct-to-metal adhesion. However, they can suffer from photooxidation and yellowing due to the presence of amines, which affects the long-term color and gloss stability of these coatings. Similar to two-component polyurethanes, these materials suffer from poor application appearance and performance if not mixed correctly by applicators, not to mention the limited pot-life and waste associated with a two-component system.
Single-component polysiloxane coatings are traditionally based on acrylic-silane polymers. These polymers are manufactured via radical polymerization of gamma-methacryloxypropyltrimethoxysilane with methyl methacrylate, hexyl acrylate or other organic monomers to form linear copolymers with pendant alkoxysilane groups. The copolymers are high in molecular weight and require significant quantities of solvent(s) to solubilize the large polymer chains, thus making it difficult to generate low VOC coatings. The pendant alkoxysilane groups are the only reactive functionalities on the copolymer, which enables the coating to be cured via moisture hydrolysis and condensation. Single-component coatings based on these polymers are available on the commercial market from several manufacturers, although they are not without their drawbacks. For instance, these coatings are slow to hydrolyze and crosslink (cure) at room temperature when not exposed to high humidity environments, and they display poor chemical resistance when not fully cured due to the low crosslink density within the coating. These issues result because the acrylic-silane copolymers in the coating contain pendent propyltrialkoxysilane groups that are inherently slow to hydrolyze and limited in quantity when compared to the non-reactive groups in the copolymer backbone. Acrylic-silane binders often possess glass transition temperatures (Tgs) above room temperature in order to provide fast dry-to-touch times (e.g., 1-3 hours), even though the crosslinking reaction between polymers is slow to occur.
Single-component moisture-curable coating compositions were disclosed in U.S. Pat. No. 6,288,198. These coatings are based on aliphatic polyisocyanate-aminosilane adducts, where greater than 70% of the isocyanate groups are reacted with an aminosilane, which is then combined with a hydrolysable silane to form a hybrid sol-gel coating. It is stated that these sol-gel coatings provide hard, abrasion-resistant and solvent-resistant surfaces, which is expected for highly-crosslinked coatings, especially those that contain small hydrolyzable silanes. However, the reported flexibility is only a 90 degree bend, not a 180 degree bend, which is the norm when referring to a highly flexible coating. Furthermore, the preferred coating dry film thickness is only 2-30 microns, which is significantly less than what is utilized for most commercial and military coatings. An additional drawback to these coating compositions are that the high content of moisture-curable silane groups within the coatings leads to a continual reduction in gloss over time as the coating post-cures with moisture.
Single-component moisture-curable coatings were also disclosed in U.S. Pat. No. 8,133,964, and are based on similar aliphatic polyisocyanate-aminosilane adducts as those discussed above (paragraph [0009]). However, these adducts are formed by reacting polyisocyanates with 2:1 or 1:2 ratios of N-substituted aminosilanes and di-substituted mono-functional amines. Reactive diluents, such as hydrolyzable silanes or polysiloxanes could also be utilized. The di-substituted mono-functional amines reduced the amount of hydrolysable silane groups on the polyisocyanate-aminosilane adduct, but the overall high concentration of moisture-curable silane groups in the coating yielded topcoats with only slightly better flexibility than coatings reported in U.S. Pat. No. 6,288,198. The coatings still provided good solvent resistance, high hardness and low VOCs. Additional drawbacks of these coatings are that the high content of moisture-curable groups leads to a continual reduction in gloss over time as the coatings post-cure with moisture, and that the use of the di-substituted mono-functional amines results in slow tack-free and dry-through times for the coatings.